The in-line filtering unit is necessary for those systems in which the main filter unit is made of material which can be carried out of the main filter by a fluid flow through the main filter unit. In such a filtering system, an additional in-line filtering unit is necessary for removing from the fluid flow any material escaping from the main filter. An additional in-line filter is useful for removing any other unwanted material which bypassed the main filter unit.
Specifically, one such fluid filtering system which can advantageously utilize the additional in-line filter is the filtering system for a swimming pool which utilizes a sand filter as its main filter element.
The swimming pool sand filter works on the same principle as the city water works. The dirty water is pumped from the swimming pool through the filter pump and then into the top portion of the sand filter. There the water is diffused or slowed down in force. The water travels through the sand bed where the dust and debris are trapped between the grains of sand. Such sand filters usually use number 20 or 30 size grain of sand. The filtered water then flows through the underdrain and back into the pool.
The in-line filtering device of the present invention removes from the water flow, any number 20 and 30 sand particles or any other debris passed by the main filter. The sand filters of today pass a few grains of sand back into the pool which are not noticeable under normal filtering conditions.
Automatic cleaning systems for swimming pools are being used in ever increasing numbers. Such automatic cleaning systems are often powered by water pressure developed by a booster pump placed between the output of the main filter and the cleaning equipment positioned in the pool. This cleaning equipment in the pool is normally made of a plastic like material. Those grains of sand passed by the main filter which are not noticeable when automatic equipment is not being used, cause great damage to the plastic like material of the automatic cleaning equipment.
When the water passes through the main filter, there are several paths by which the silica media can escape through the filter underdrain unit and be deposited in the return fluid flow to the swimming pool. Debris and sand which can pass through or around the main filter occurs under many circumstances. For example, after a severe dirt storm, the pool is, of course, extremely dirty. As the automatic cleaning system is working to remove this debris, the cleaning system agitates the water to hold the debris in suspension so that the main drain and the skimmer of the swimming pool can draw the debris out of the pool. The main swimming pool filter becomes dirty and consequently the water flow through the filter slows. When the water flow slows, the automatic cleaning system is still operating and pushes debris and sand and dirt down into the main drain of the pool. As the water flow slows down, this debris stays in the main drain pipe and then when the homeowner backwashes the main filtering system, this debris then is caught in the under drain of the filter and is trapped there. When the homeowner turns the main filtering system back on to filter, then this debris is put back into the main return line back to the pool in which the automatic cleaning system plumbing is attached. This dirt and debris and sand is caught into the main automatic cleaning system.
The debris trapped in the main drain of the swimming pool as just described can also be trapped during backwash in a diatomaceous earth filter.
One of the common occurrences in maintaining a swimming pool is the cleaning of the main filter. This cleaning of the filter is called backwashing. When backwashing either a sand filter or a diatomaceous earth filter, material from the primary filtering unit itself can escape from the inside of the filter into the fluid return line to the pool.
In filters presently in use, the filtering material from which the under drain is made of is of such porous character that sand grains are able to pass therethrough during normal operation. Although the number of sand grains passing through is of very small amount, it will be described hereinafter how these very few grains of sand or for that matter individual grains of sand can damage other expensive equipment associated in maintenance of the swimming pool.
Filtering systems, especially high rate sand filters, are manufactured to include an internal bleed line for passing air out of the interior of the sand filter. The internal bleed line is connected to the output of the filter. In effect the internal bleed line bypasses the filter. If any debris enters the internal bleed line, that debris would bypass the primary filter and be returned to the pool by the return line. Since the cleaning equipment is powered by the return line, this debris would pass through the cleaning equipment.
Debris enters the bleeder line in the following manner. During the filtering operation, the debris and sand in the upper portion of the high rate sand filter swirl and float about and circulate within the entire upper portion of the high rate sand filter. During this circulation, individual particles of sand and/or debris can find their way into the bleeder line which leads to the fluid return line to the swimming pool. This is an additional source of sand and/or debris for the pool return line.
The in-line filter unit is placed in the fluid return line to the swimming pool and removes the sand and debris before they reach the automatic cleaning unit and cause damage to the automatic cleaning unit.